Normal kinetics of intestinal glucose absorption in the absence of GLUT2: evidence for a transport pathway requiring glucose phosphorylation and transfer into the endoplasmic reticulum

Effects of Commonly used Surfactants, Poloxamer 188 and Tween 80, on the Drug Transport Capacity of Intestinal Glucose Transporters

Some glycoside drugs can be transported through intestinal glucose transporters (IGTs). The surfactants used in oral drug preparations can affect the function of transporter proteins. This study aimed to investigate the effect of commonly used surfactants, Poloxamer 188 and Tween 80, on the drug transport capacity of IGTs. Previous studies have shown that gastrodin is the optimal drug substrate for IGTs. Gastrodin was used as a probe drug to evaluate the effect of these two surfactants on intestinal absorption in SD rats through pharmacokinetic and in situ single-pass intestinal perfusion. Then, the effects of the two surfactants on the expression of glucose transporters and tight-junction proteins were examined using RT-PCR and western blotting. Additionally, the effect of surfactants on intestinal permeability was evaluated through hematoxylin-eosin staining. The results found that all experimental for Poloxamer 188 (0.5%, 2.0% and 8.0%) and Tween 80 (0.1% and 2.0%) were not significantly different from those of the blank group. However, the AUC(0-∞) of gastrodin increased by approximately 32% when 0.5% Tween 80 was used. The changes in IGT expression correlated with the intestinal absorption of gastrodin. A significant increase in the expression of IGTs was observed at 0.5% Tween 80. In conclusion, Poloxamer 188 had minimal effect on the drug transport capacity of IGTs within the recommended limits of use. However, the expression of IGTs increased in response to 0.5% Tween 80, which significantly enhanced the drug transport capacity of IGTs. However, 0.1% and 2.0% Tween 80 had no significant effect.

Physiological functions of glucose transporter-2: From cell physiology to links with diabetes mellitus

Glucose is a sugar crucial for human health since it participates in many biochemical reactions. It produces adenosine 5'-triphosphate (ATP) and nucleosides through glucose metabolic and pentose phosphate pathways. These processes require many transporter proteins to assist in transferring glucose across cells, and the most notable ones are glucose transporter-2 (GLUT-2) and sodium/glucose cotransporter 1 (SGLT1). Glucose enters small intestinal epithelial cells from the intestinal lumen by crossing the brush boundary membrane via the SGLT1 cotransporter. It exits the cells by traversing the basolateral membrane through the activity of the GLUT-2 transporter, supplying energy throughout the body. Dysregulation of these glucose transporters is involved in the pathogenesis of several metabolic diseases, such as diabetes. Natural loss of GLUT-2 or its downregulation causes abnormal blood glucose concentrations in the body, such as fasting hypoglycemia and glucose tolerance. Therefore, understanding GLUT-2 physiology is necessary for exploring the mechanisms of diabetes and targeted treatment development. This article reviews how the apical GLUT-2 transporter maintains normal physiological functions of the human body and the adaptive changes this transporter produces under pathological conditions such as diabetes.

Transcellular Barriers to Glucose Delivery in the Body

Glucose is the universal fuel of most mammalian cells, and it is largely replenished through dietary intake. Glucose availability to tissues is paramount for the maintenance of homeostatic energetics and, hence, supply should match demand by the consuming organs. In its journey through the body, glucose encounters cellular barriers for transit at the levels of the absorbing intestinal epithelial wall, the renal epithelium mediating glucose reabsorption, and the tight capillary endothelia (especially in the brain). Glucose transiting through these cellular barriers must escape degradation to ensure optimal glucose delivery to the bloodstream or tissues. The liver, which stores glycogen and generates glucose de novo, must similarly be able to release it intact to the circulation. We present the most up-to-date knowledge on glucose handling by the gut, liver, brain endothelium, and kidney, and discuss underlying molecular mechanisms and open questions. Diseases associated with defects in glucose delivery and homeostasis are also briefly addressed. We propose that the universal problem of sparing glucose from catabolism in favor of translocation across the barriers posed by epithelia and endothelia is resolved through common mechanisms involving glucose transfer to the endoplasmic reticulum, from where glucose exits the cells via unconventional cellular mechanisms.

Endogenous Glucose Production in Patients With Glycogen Storage Disease Type Ia Estimated by Oral D-[6,6-2H2]-glucose

CONTEXT

Glycogen storage disease type Ia (GSDIa) is an inborn metabolic disorder characterized by impaired endogenous glucose production (EGP). Monitoring of patients with GSDIa is prioritized because of ongoing treatment developments. Stable isotope tracers may enable reliable EGP monitoring.

OBJECTIVE

The aim of this study was to prospectively assess the rate of appearance of endogenous glucose into the bloodstream (Ra) in patients with GSDIa after a single oral D-[6,6-2H2]-glucose dose.

METHODS

Ten adult patients with GSDIa and 10 age-, sex-, and body mass index-matched healthy volunteers (HVs) were enrolled. For each participant, 3 oral glucose tracer tests were performed: (1) preprandial/fasted, (2) postprandial, and (3) randomly fed states. Dried blood spots were collected before D-[6,6-2H2]-glucose administration and 10, 20, 30, 40, 50, 60, 75, 90, and 120 minutes thereafter.

RESULTS

Glucose Ra in fasted HVs was consistent with previously reported data. The time-averaged glucose Ra was significantly higher in (1) preprandial/fasted patients with GSDIa than HV and (2) postprandial HV compared with fasted HV(P < .05). A progressive decrease in glucose Ra was observed in preprandial/fasted patients with GSDIa; the change in glucose Ra time-course was directly correlated with the change in capillary glucose (P < .05).

CONCLUSION

This is the first study to quantify glucose Ra in patients with GSDIa using oral D-[6,6-2H2] glucose. The test can reliably estimate EGP under conditions in which fasting tolerance is unaffected but does not discriminate between relative contributions of EGP (eg, liver, kidney) and exogenous sources (eg, dietary cornstarch). Future application is warranted for longitudinal monitoring after novel genome based treatments in patients with GSDIa in whom nocturnal dietary management can be discontinued.

Trodusquemine (MSI-1436) Restores Metabolic Flexibility and Mitochondrial Dynamics in Insulin-Resistant Equine Hepatic Progenitor Cells (HPCs)

Equine metabolic syndrome (EMS) is a significant global health concern in veterinary medicine. There is increasing interest in utilizing molecular agents to modulate hepatocyte function for potential clinical applications. Recent studies have shown promising results in inhibiting protein tyrosine phosphatase (PTP1B) to maintain cell function in various models. In this study, we investigated the effects of the inhibitor Trodusquemine (MSI-1436) on equine hepatic progenitor cells (HPCs) under lipotoxic conditions. We examined proliferative activity, glucose uptake, and mitochondrial morphogenesis. Our study found that MSI-1436 promotes HPC entry into the cell cycle and protects them from palmitate-induced apoptosis by regulating mitochondrial dynamics and biogenesis. MSI-1436 also increases glucose uptake and protects HPCs from palmitate-induced stress by reorganizing the cells' morphological architecture. Furthermore, our findings suggest that MSI-1436 enhances 2-NBDG uptake by increasing the expression of SIRT1, which is associated with liver insulin sensitivity. It also promotes mitochondrial dynamics by modulating mitochondria quantity and morphotype as well as increasing the expression of PINK1, MFN1, and MFN2. Our study provides evidence that MSI-1436 has a positive impact on equine hepatic progenitor cells, indicating its potential therapeutic value in treating EMS and insulin dysregulation.

Review: A species comparison of the kinetic homogeneous and heterogeneous organization of sodium-dependent glucose transport systems along the intestine

The targeted use of carbohydrates by feed and food industries to create balanced and cost-effective diets has generated a tremendous amount of research in carbohydrate digestion and absorption in different species. Specifically, this research has led us to a larger observation that identified different organizations of intestinal sodium-dependent glucose absorption across species, which has not been previously collated and reviewed. Thus, this review will compare the kinetic segregation of sodium-dependent glucose transport across the intestine of different species, which we have termed either homogeneous or heterogeneous systems. For instance, the pig follows a heterogeneous system of sodium-dependent glucose transport with a high-affinity, super-low-capacity (Ha/sLc) in the jejunum, and a high-affinity, super-high-capacity (Ha/sHc) in the ileum. This is achieved by multiple sodium-dependent glucose transporters contributing to each segment. In contrast, tilapia have a homogenous system characterized by high-affinity, high-capacity (Ha/Hc) throughout the intestine. Additionally, we are the first to report glucose transporter patterns across species presented from vertebrates to invertebrates. Finally, other kinetic transport systems are briefly covered to illustrate possible contributions/modulations to sodium-dependent glucose transporter organization. Overall, we present a new perspective on the organization of glucose absorption along the intestinal tract.

The resolution of phagosomes

Phagocytosis is a fundamental immunobiological process responsible for the removal of harmful particulates. While the number of phagocytic events achieved by a single phagocyte can be remarkable, exceeding hundreds per day, the same phagocytic cells are relatively long-lived. It should therefore be obvious that phagocytic meals must be resolved in order to maintain the responsiveness of the phagocyte and to avoid storage defects. In this article, we discuss the mechanisms involved in the resolution process, including solute transport pathways and membrane traffic. We describe how products liberated in phagolysosomes support phagocyte metabolism and the immune response. We also speculate on mechanisms involved in the redistribution of phagosomal metabolites back to circulation. Finally, we highlight the pathologies owed to impaired phagosome resolution, which range from storage disorders to neurodegenerative diseases.

The role of GLUT2 in glucose metabolism in multiple organs and tissues

The glucose transporter family has an important role in the initial stage of glucose metabolism; Glucose transporters 2 (GLUTs, encoded by the solute carrier family 2, SLC2A genes) is the major glucose transporter in β-cells of pancreatic islets and hepatocytes but is also expressed in the small intestine, kidneys, and central nervous system; GLUT2 has a relatively low affinity to glucose. Under physiological conditions, GLUT2 transports glucose into cells and allows the glucose concentration to reach balance on the bilateral sides of the cellular membrane; Variation of GLUT2 is associated with various endocrine and metabolic disorders; In this study, we discussed the role of GLUT2 in participating in glucose metabolism and regulation in multiple organs and tissues and its effects on maintaining glucose homeostasis.

Regulatory mechanisms of glucose absorption in the mouse proximal small intestine during fasting and feeding

Fasting is known to alter the function of various organs and the mechanisms of glucose metabolism, which affect health outcomes and slow aging. However, it remains unclear how fasting and feeding affects glucose absorption function in the small intestine. We studied the effects of the fasting and feeding on glucose-induced short-circuit current (I_sc) in vitro using an Ussing chamber technique. Glucose-induced I_sc by SGLT1 was observed in the ileum, but little or no I_sc was observed in the jejunum in ad libitum-fed mice. However, in mice fasted for 24-48 h, in addition to the ileum, robust glucose-induced I_sc was observed over time in the jejunum. The expression of SGLT1 in the brush border membranes was significantly decreased in the jejunum under fed conditions compared to 48 h fasting, as analyzed by western blotting. Additionally, when mice were fed a 60% high glucose diet for 3 days, the increase in glucose-induced I_sc was observed only in the ileum, and totally suppressed in the jejunum. An increase in Na⁺ permeability between epithelial cells was concomitantly observed in the jejunum of fasted mice. Transepithelial glucose flux was assessed using a non-metabolizable glucose analog, ¹⁴C-methyl α-D-glucopyranoside glucose (MGP). Regardless of whether fed or fasted, no glucose diffusion mechanism was observed. Fasting increased the SGLT1-mediated MGP flux in the jejunum. In conclusion, segment-dependent up- and down-regulation mechanisms during fasting and feeding are important for efficient glucose absorption once the fast is broken. Additionally, these mechanisms may play a crucial role in the small intestine's ability to autoregulate glucose absorption, preventing acute hyperglycemia when large amounts of glucose are ingested.

Attenuated glucose uptake promotes catabolic metabolism through activated AMPK signaling and impaired insulin signaling in zebrafish

Glucose metabolism in fish remains a controversial area of research as many fish species are traditionally considered glucose-intolerant. Although energy homeostasis remodeling has been observed in fish with inhibited fatty acid β-oxidation (FAO), the effects and mechanism of the remodeling caused by blocked glucose uptake remain poorly understood. In this study, we blocked glucose uptake by knocking out glut2 in zebrafish. Intriguingly, the complete lethality, found in Glut2-null mice, was not observed in glut2^-/- zebrafish. Approxiamately 30% of glut2^-/- fish survived to adulthood and could reproduce. The maternal zygotic mutant glut2 (MZglut2) fish exhibited growth retardation, decreased blood and tissue glucose levels, and low locomotion activity. The decreased pancreatic β-cell numbers and insulin expression, as well as liver insulin receptor a (insra), fatty acid synthesis (chrebp, srebf1, fasn, fads2, and scd), triglyceride synthesis (dgat1a), and muscle mechanistic target of rapamycin kinase (mtor) of MZglut2 zebrafish, suggest impaired insulin-dependent anabolic metabolism. Upregulated expression of lipolysis (atgl and lpl) and FAO genes (cpt1aa and cpt1ab) in the liver and proteolysis genes (bckdk, glud1b, and murf1a) in muscle were observed in the MZglut2 zebrafish, as well as elevated levels of P-AMPK proteins in both the liver and muscle, indicating enhanced catabolic metabolism associated with AMPK signaling. In addition, decreased amino acids and elevated carnitines of the MZglut2 zebrafish supported the decreased protein and lipid content of the whole fish. In summary, we found that blocked glucose uptake impaired insulin signaling-mediated anabolism via β-cell loss, while AMPK signaling-mediated catabolism was enhanced. These findings reveal the mechanism of energy homeostasis remodeling caused by blocked glucose uptake, which may be a potential strategy for adapting to low glucose levels.

Attempts to limit sporulation in the probiotic strain Bacillus subtilis BG01-4TM through mutation accumulation and selection

The use of bacterial spores in probiotics over viable loads of bacteria has many advantages, including the durability of spores, which allows spore-based probiotics to effectively traverse the various biochemical barriers present in the gastrointestinal tract. However, the majority of spore-based probiotics developed currently aim to treat adults, and there is a litany of differences between the adult and infant intestinal systems, including the immaturity and low microbial species diversity observed within the intestines of infants. These differences are only further exacerbated in premature infants with necrotizing enterocolitis (NEC) and indicates that what may be appropriate for an adult or even a healthy full-term infant may not be suited for an unhealthy premature infant. Complications from using spore-based probiotics for premature infants with NEC may involve the spores remaining dormant and adhering to the intestinal epithelia, the out-competing of commensal bacteria by spores, and most importantly the innate antibiotic resistance of spores. Also, the ability of Bacillus subtilis to produce spores under duress may result in less B. subtilis perishing within the intestines and releasing membrane branched-chain fatty acids. The isolate B. subtilis BG01-4^TM is a proprietary strain developed by Vernx Biotechnology through accumulating mutations within the BG01-4^TM genome in a serial batch culture. Strain BG01-4^TM was provided as a non-spore-forming B. subtilis , but a positive sporulation status for BG01-4^TM was confirmed through in vitro testing and suggested that selection for the sporulation defective genes could occur within an environment that would select against sporulation. The durability of key sporulation genes was ratified in this study, as the ability of BG01-4^TM to produce spores was not eliminated by the attempts to select against sporulation genes in BG01-4^TM by the epigenetic factors of high glucose and low pH. However, a variation in the genes in isolate BG01-4-8 involved in the regulation of sporulation is believed to have occurred during the mutation selection from the parent strain BG01-4^TM. An alteration in selected sporulation regulation genes is expected to have occurred from BG01-4^TM to BG01-4-8, with BG01-4-8 producing spores within 24 h, ~48 h quicker than BG01-4^TM.

Intestinal Farnesoid X Receptor Modulates Duodenal Surface Area but Does Not Control Glucose Absorption in Mice

Bile acids facilitate the intestinal absorption of dietary lipids and act as signalling molecules in the maintenance of metabolic homeostasis. Farnesoid X receptor (FXR) is a bile acid-responsive nuclear receptor involved in bile acid metabolism, as well as lipid and glucose homeostasis. Several studies have suggested a role of FXR in the control of genes regulating intestinal glucose handling. We applied a novel dual-label glucose kinetic approach in intestine-specific FXR^-/- mice (iFXR-KO) to directly assess the role of intestinal FXR in glucose absorption. Although iFXR-KO mice showed decreased duodenal expression of hexokinase 1 (Hk1) under obesogenic conditions, the assessment of glucose fluxes in these mice did not show a role for intestinal FXR in glucose absorption. FXR activation with the specific agonist GS3972 induced Hk1, yet the glucose absorption rate remained unaffected. FXR activation increased the duodenal villus length in mice treated with GS3972, while stem cell proliferation remained unaffected. Accordingly, iFXR-KO mice on either chow, short or long-term HFD feeding displayed a shorter villus length in the duodenum compared to wild-type mice. These findings indicate that delayed glucose absorption reported in whole-body FXR^-/- mice is not due to the absence of intestinal FXR. Yet, intestinal FXR does have a role in the small intestinal surface area.

Diagnostic and prognostic significance of SLC50A1 expression in patients with primary early breast cancer

There is a lack of validated biomarkers for the diagnosis of early breast cancer (EBC). The current study aimed to determine the diagnostic and prognostic value of solute carrier family 50 member 1 (SLC50A1) in patients with EBC. Therefore, 123 patients with EBC, 30 patients with benign breast disease (BBD) and 26 healthy controls (HCs) were recruited. The serum levels of SLC50A1 in paired sera of 40 postoperative patients were assessed by ELISA. Immunohistochemical staining for SLC50A1 was performed in surgical tissue derived from 83 patients with EBC and 30 patients with BBD. mRNA expression of SLC50A1 and its diagnostic and prognostic value in patients with EBC was evaluated using an RNA-sequencing database. The results showed that serum levels of SLC50A1 in patients with EBC were significantly higher compared with those in patients with BBD and HCs (both P<0.001). Additionally, receiver operating characteristic curve analysis revealed that the serum levels of SLC50A1 distinguished patients with EBC from patients with BBD and HCs with a sensitivity of 76.42% and specificity of 76.79% [area under the curve (AUC)=0.783; P<0.001]. The diagnostic value of SLC50A1 was significantly greater than that of carcinoembryonic (P<0.005) and carbohydrate antigen 15-3 (P<0.029). Furthermore, the number of SLC50A1 positive cells significantly increased in tissue of patients with EBC compared with patients with BBD (P<0.001). A positive association between serum levels of SLC50A1 and its expression in tissue samples was observed in patients with EBC (ρ=0.700; P<0.001). Additionally, bioinformatics analysis verified the diagnostic value of SLC50A1, with an AUC of 0.983 (P<0.001). Multivariate analysis demonstrated that SLC50A1 was an independent prognostic factor in patients with EBC with a hazard ratio of 1.917 (P=0.013). These findings indicated that SLC50A1 may be a potential diagnostic biomarker for primary EBC and that SLC50A1 upregulation may be associated with unfavorable prognosis in patients with EBC.

Dynamic glucose uptake, storage, and release by human microvascular endothelial cells

Endothelia determine blood-to-tissue solute delivery, yet glucose transit is poorly understood. To illuminate mechanisms, we tracked [³H]-2-deoxyglucose (2-DG) in human adipose-tissue microvascular endothelial cells. 2-DG uptake was largely facilitated by the glucose transporters GLUT1 and GLUT3. Once in the cytosol, >80% of 2-DG became phosphorylated and ∼20% incorporated into glycogen, suggesting that transported glucose is readily accessible to cytosolic enzymes. Interestingly, a fraction of intracellular 2-DG was released over time (15-20% over 30 min) with slower kinetics than for uptake, involving GLUT3. In contrast to intracellular 2-DG, the released 2-DG was largely unphosphorylated. Glucose release involved endoplasmic reticulum-resident translocases/phosphatases and was stimulated by adrenaline, consistent with participation of glycogenolysis and glucose dephosphorylation. Surprisingly, the fluorescent glucose derivative 2-NBD-glucose (2-NBDG) entered cells largely via fluid phase endocytosis and exited by recycling. 2-NBDG uptake was insensitive to GLUT1/GLUT3 inhibition, suggesting poor influx across membranes. 2-NBDG recycling, but not 2-DG efflux, was sensitive to N-ethyl maleimide. In sum, by utilizing radioactive and fluorescent glucose derivatives, we identified two parallel routes of entry: uptake into the cytosol through dedicated glucose transporters and endocytosis. This reveals the complex glucose handling by endothelial cells that may contribute to glucose delivery to tissues.

Enteral glucose, absorbed and metabolized, potently enhances mesenteric lymph flow in chow- and high-fat-fed rats

A portion of absorbed dietary triglycerides (TG) is retained in the intestine after the postprandial period, within intracellular and extracellular compartments. This pool of TG can be mobilized in response to several stimuli, including oral glucose. The objective of this study was to determine whether oral glucose must be absorbed and metabolized to mobilize TG in rats and whether high-fat feeding, a model of insulin resistance, alters the lipid mobilization response to glucose. Lymph flow, TG concentration, TG output, and apolipoprotein B48 (apoB48) concentration and output were assessed after an intraduodenal lipid bolus in rats exposed to the following intraduodenal administrations 5 h later: saline (placebo), glucose, 2-deoxyglucose (2-DG, absorbed but not metabolized), or glucose + phlorizin (intestinal glucose absorption inhibitor). Glucose alone, but not 2-DG or glucose + phlorizin treatments, stimulated lymph flow, TG output, and apoB48 output compared with placebo. The effects of glucose in high-fat-fed rats were similar to those in chow-fed rats. In conclusion, glucose must be both absorbed and metabolized to enhance lymph flow and intestinal lipid mobilization. This effect is qualitatively and quantitatively similar in high-fat- and chow-fed rats. The precise signaling mechanism whereby enteral glucose enhances lymph flow and mobilizes enteral lipid remains to be determined.NEW & NOTEWORTHY Glucose potently enhances mesenteric lymph flow in chow- and high-fat-fed rats. The magnitude of glucose effect on lymph flow is no different in chow- and high-fat-fed rats. Glucose must be absorbed and metabolized to enhance lymph flow and mobilize intestinal lipid.

An Exploratory Study of the Role of Dietary Proteins in the Regulation of Intestinal Glucose Absorption

Several studies have demonstrated that high protein diets improve glucose homeostasis. Nevertheless, the mechanisms underlying this effect remain elusive. This exploratory study aims to screen and compare the acute effects of dietary proteins from different sources on intestinal glucose absorption. Six dietary proteins from various sources were thus selected and digested thanks to the INFOGEST static gastrointestinal digestion protocol. The digested proteins were able to decrease intestinal glucose absorption in vitro and ex vivo. Moreover, acute ingestion of casein and fish gelatin led to improved glucose tolerance in Wistar rats without significant effect on insulin secretion. In parallel, GLUT2 mRNA expression in enterocytes was decreased following short-term incubation with some of the digested proteins. These results strengthen the evidence that digested protein-derived peptides and amino acids are key regulators of glucose homeostasis and highlight their role in intestinal glucose absorption.

Molecular aspects of fructose metabolism and metabolic disease

Excessive sugar consumption is increasingly considered as a contributor to the emerging epidemics of obesity and the associated cardiometabolic disease. Sugar is added to the diet in the form of sucrose or high-fructose corn syrup, both of which comprise nearly equal amounts of glucose and fructose. The unique aspects of fructose metabolism and properties of fructose-derived metabolites allow for fructose to serve as a physiological signal of normal dietary sugar consumption. However, when fructose is consumed in excess, these unique properties may contribute to the pathogenesis of cardiometabolic disease. Here, we review the biochemistry, genetics, and physiology of fructose metabolism and consider mechanisms by which excessive fructose consumption may contribute to metabolic disease. Lastly, we consider new therapeutic options for the treatment of metabolic disease based upon this knowledge.

Plant SWEETs: from sugar transport to plant-pathogen interaction and more unexpected physiological roles

Sugars Will Eventually be Exported Transporters (SWEETs) have important roles in numerous physiological mechanisms where sugar efflux is critical, including phloem loading, nectar secretion, seed nutrient filling, among other less expected functions. They mediate low affinity and high capacity transport, and in angiosperms this family is composed by 20 paralogs on average. As SWEETs facilitate the efflux of sugars, they are highly susceptible to hijacking by pathogens, making them central players in plant-pathogen interaction. For instance, several species from the Xanthomonas genus are able to upregulate the transcription of SWEET transporters in rice (Oryza sativa), upon the secretion of transcription-activator-like effectors. Other pathogens, such as Botrytis cinerea or Erysiphe necator, are also capable of increasing SWEET expression. However, the opposite behavior has been observed in some cases, as overexpression of the tonoplast AtSWEET2 during Pythium irregulare infection restricted sugar availability to the pathogen, rendering plants more resistant. Therefore, a clear-cut role for SWEET transporters during plant-pathogen interactions has so far been difficult to define, as the metabolic signatures and their regulatory nodes, which decide the susceptibility or resistance responses, remain poorly understood. This fuels the still ongoing scientific question: what roles can SWEETs play during plant-pathogen interaction? Likewise, the roles of SWEET transporters in response to abiotic stresses are little understood. Here, in addition to their relevance in biotic stress, we also provide a small glimpse of SWEETs importance during plant abiotic stress, and briefly debate their importance in the particular case of grapevine (Vitis vinifera) due to its socioeconomic impact.

Polyphenolic inhibition of enterocytic starch digestion enzymes and glucose transporters for managing type 2 diabetes may be reduced in food systems

With the current global surge in diabetes cases, there is a growing interest in slowing and managing diabetes and its effects. While there are medications that can be used, they have adverse side effects such as hypoglycemia and weight gain. To overcome these problems, bioactive compounds commonly found in fruits, vegetables and cereal grains are used to slow starch digestion and transport of simple sugars across the intestinal epithelia thereby reducing plasma blood glucose spike. These effects are achieved through inhibition of amylases, glucosidases and glucose transporters present in the gastrointestinal tract and brush boarder membrane. The extent of inhibition by polyphenols is dependent on molecular structure, doses and food matrix. Glycemic lowering effect of polyphenols have been demonstrated both in in vivo and in vitro studies. However, when these compounds are incorporated in food systems, they can interact with other polymers in the food matrix leading to lesser inhibition of digestion and/or glucose transporters compared to isolated or pure compounds as often witnessed in most in vitro studies.

Expression of glucose transporters in human neurodegenerative diseases

The central nervous system (CNS) plays an important role in the human body. It is involved in the receive, store and participation in information retrieval. It can use several substrates as a source of energy, however, the main source of energy is glucose. Cells of the central nervous system need a continuous supply of energy, therefore, transport of glucose into these cells is very important. There are three distinct families of glucose transporters: sodium-independent glucose transporters (GLUTs), sodium-dependent glucose cotransporters (SGLTs), and uniporter, SWEET protein. In the human brain only GLUTs and SGLTs were detected. In neurodegenerative diseases was observed hypometabolism of glucose due to decreased expression of glucose transporters, in particular GLUT1 and GLUT3. On the other hand, animal studies revealed, that increased levels of these glucose transporters, due to for example by the increased copy number of SLC2A genes, may have a beneficial effect and may be a targeted therapy in the treatment of patients with AD, HD and PD.

Long-Term Dietary Changes in Subjects with Glucose Galactose Malabsorption Secondary to Biallelic Mutations of SLC5A1

BACKGROUND

Glucose galactose malabsorption (GGM) is a congenital diarrheal disorder of intestinal Na+/glucose cotransport (SGLT1/SLC5A1). The required glucose and galactose-restricted diet has been well described in infancy, but long-term nutrition follow-up is limited.

AIM

To perform a comprehensive nutritional assessment on a cohort of patients with GGM to gain insights into the consumption patterns within the population.

METHODS

A cross-sectional study examining dietary intake of a GGM cohort using prospective food records. The calories and nutrients of all foods, beverages, and condiments were analyzed with descriptive statistics and compared to intake patterns of age- and sex-matched NHANES groups.

RESULTS

The six patients were 0.7-26 years old. Whole foods and vegetable fats were major parts of the diet, while dairy and added sweeteners were restricted. Compared to typical US intakes, mean macronutrient distribution was 88th percentile from fat, 18th percentile from carbohydrates, and 78th percentile from protein. Fructose consumption, as a proportion of total sugar intake, decreased with age, from 86.1 to 50.4%. Meanwhile, glucose consumption increased with age, from 13.8 to 48.6% of sugar intake. However, the actual amount of glucose consumed remained low, equivalent to 4th percentile of US consumption level. Galactose intake was marginal throughout life.

CONCLUSIONS

A GGM diet is a high-fat and high-protein/low-carbohydrate diet that is rich in fruits and vegetables but limited in dairy and added sugar. Relatively less fructose but more glucose is incorporated into the diet with age. Future studies should investigate the effects of the GGM diet on gut microbiome and long-term health.

A comprehensive overview of substrate specificity of glycoside hydrolases and transporters in the small intestine : "A gut feeling"

The human body is able to process and transport a complex variety of carbohydrates, unlocking their nutritional value as energy source or as important building block. The endogenous glycosyl hydrolases (glycosidases) and glycosyl transporter proteins located in the enterocytes of the small intestine play a crucial role in this process and digest and/or transport nutritional sugars based on their structural features. It is for these reasons that glycosidases and glycosyl transporters are interesting therapeutic targets to combat sugar related diseases (such as diabetes) or to improve drug delivery. In this review we provide a detailed overview focused on the molecular structure of the substrates involved as a solid base to start from and to fuel research in the area of therapeutics and diagnostics.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Sodium-coupled glucose transport, the SLC5 family, and therapeutically relevant inhibitors: from molecular discovery to clinical application

Sodium glucose transporters (SGLTs) belong to the mammalian solute carrier family SLC5. This family includes 12 different members in human that mediate the transport of sugars, vitamins, amino acids, or smaller organic ions such as choline. The SLC5 family belongs to the sodium symporter family (SSS), which encompasses transporters from all kingdoms of life. It furthermore shares similarity to the structural fold of the APC (amino acid-polyamine-organocation) transporter family. Three decades after the first molecular identification of the intestinal Na+-glucose cotransporter SGLT1 by expression cloning, many new discoveries have evolved, from mechanistic analysis to molecular genetics, structural biology, drug discovery, and clinical applications. All of these advances have greatly influenced physiology and medicine. While SGLT1 is essential for fast absorption of glucose and galactose in the intestine, the expression of SGLT2 is largely confined to the early part of the kidney proximal tubules, where it reabsorbs the bulk part of filtered glucose. SGLT2 has been successfully exploited by the pharmaceutical industry to develop effective new drugs for the treatment of diabetic patients. These SGLT2 inhibitors, termed gliflozins, also exhibit favorable nephroprotective effects and likely also cardioprotective effects. In addition, given the recent finding that SGLT2 is also expressed in tumors of pancreas and prostate and in glioblastoma, this opens the door to potential new therapeutic strategies for cancer treatment by specifically targeting SGLT2. Likewise, further discoveries related to the functional association of other SGLTs of the SLC5 family to human pathologies will open the door to potential new therapeutic strategies. We furthermore hope that the herein summarized information about the physiological roles of SGLTs and the therapeutic benefits of the gliflozins will be useful for our readers to better understand the molecular basis of the beneficial effects of these inhibitors, also in the context of the tubuloglomerular feedback (TGF), and the renin-angiotensin system (RAS). The detailed mechanisms underlying the clinical benefits of SGLT2 inhibition by gliflozins still warrant further investigation that may serve as a basis for future drug development.

Fanconi-Bickel Syndrome: A Review of the Mechanisms That Lead to Dysglycaemia

Accumulation of glycogen in the kidney and liver is the main feature of Fanconi-Bickel Syndrome (FBS), a rare disorder of carbohydrate metabolism inherited in an autosomal recessive manner due to SLC2A2 gene mutations. Missense, nonsense, frame-shift (fs), in-frame indels, splice site, and compound heterozygous variants have all been identified in SLC2A2 gene of FBS cases. Approximately 144 FBS cases with 70 different SLC2A2 gene variants have been reported so far. SLC2A2 encodes for glucose transporter 2 (GLUT2) a low affinity facilitative transporter of glucose mainly expressed in tissues playing important roles in glucose homeostasis, such as renal tubular cells, enterocytes, pancreatic β-cells, hepatocytes and discrete regions of the brain. Dysfunctional mutations and decreased GLUT2 expression leads to dysglycaemia (fasting hypoglycemia, postprandial hyperglycemia, glucose intolerance, and rarely diabetes mellitus), hepatomegaly, galactose intolerance, rickets, and poor growth. The molecular mechanisms of dysglycaemia in FBS are still not clearly understood. In this review, we discuss the physiological roles of GLUT2 and the pathophysiology of mutants, highlight all of the previously reported SLC2A2 mutations associated with dysglycaemia, and review the potential molecular mechanisms leading to dysglycaemia and diabetes mellitus in FBS patients.

Intestinal Absorption and Tissue Distribution of Aza-Sugars from Mulberry Leaves and Evaluation of Their Transport by Sugar Transporters

Mulberry leaves are rich in aza-sugars, particularly 1-deoxynojirimycin (DNJ), fagomine, and 2-O-α-d-galactopyranosyl-1-deoxynojirimycin (GAL-DNJ), which have antidiabetes and antiobesity properties. To help us understand the mechanisms of action of aza-sugars, pharmacokinetic studies are necessary. Therefore, in this study, we evaluated and compared the absorption and organ distribution of these aza-sugars in rats. Following oral intake, DNJ exhibited the highest plasma concentration followed by fagomine and GAL-DNJ. Meanwhile, similar amounts of DNJ and fagomine were present in organs, while GAL-DNJ was hardly detected, suggesting the diversity in absorption and distribution characteristics of these aza-sugars. We then investigated the role of the sodium-glucose cotransporter and the glucose transporter (GLUT) in the transport of aza-sugars and found that both are involved in DNJ transport, while transport of fagomine is solely facilitated by the GLUT. These findings provide insight into the bioavailability and bioactive mechanisms of these aza-sugars.

Jejunal Insulin Signalling Is Increased in Morbidly Obese Subjects with High Insulin Resistance and Is Regulated by Insulin and Leptin

Little is known about the jejunal insulin signalling pathways in insulin resistance/diabetes states and their possible regulation by insulin/leptin. We study in jejunum the relation between insulin signalling and insulin resistance in morbidly obese subjects with low (MO-low-IR) or with high insulin resistance (MO-high-IR), and with type 2 diabetes treated with metformin (MO-metf-T2DM)), and the effect of insulin/leptin on intestinal epithelial cells (IEC). Insulin receptor substrate-1 (IRS1) and the catalytic p110β subunit (p110β) of phosphatidylinositol 3-kinase (PI3K) were higher in MO-high-IR than in MO-low-IR. The regulatory p85α subunit of PI3K (p85α)/p110β ratio was lower in MO-high-IR and MO-metf-T2DM than in MO-low-IR. Akt-phosphorylation in Ser473 was reduced in MO-high-IR compared with MO-low-IR. IRS1 and p110-β were associated with insulin and leptin levels. The improvement of body mass index (BMI) and HOMA-IR (homeostasis model assessment of insulin resistance index) after bariatric surgery was associated with a higher IRS1 and a lower p85α/p110β ratio. IEC (intestinal epithelial cells) incubation with a high glucose + insulin dose produced an increase of p85α and p110β. High dose of leptin produced an increase of IRS1, p85α and p110β. In conclusion, despite the existence of insulin resistance, the jejunal expression of genes involved in insulin signalling was increased in MO-high-IR. Their expressions were regulated mainly by leptin. IRS1 and p85α/p110β ratio was associated with the evolution of insulin resistance after bariatric surgery.

The median eminence as the hypothalamic area involved in rapid transfer of glucose to the brain: functional and cellular mechanisms

Our data proposes that glucose is transferred directly to the cerebrospinal fluid (CSF) of the hypothalamic ventricular cavity through a rapid "fast-track-type mechanism" that would efficiently stimulate the glucosensing areas. This mechanism would occur at the level of the median eminence (ME), a periventricular hypothalamic zone with no blood-brain barrier. This "fast-track" mechanism would involve specific glial cells of the ME known as β2 tanycytes that could function as "inverted enterocytes," expressing low-affinity glucose transporters GLUT2 and GLUT6 in order to rapidly transfer glucose to the CSF. Due to the large size of tanycytes, the presence of a high concentration of mitochondria and the expression of low-affinity glucose transporters, it would be expected that these cells accumulate glucose in the endoplasmic reticulum (ER) by sequestering glucose-6-phosphate (G-6-P), in a similar way to that recently demonstrated in astrocytes. Glucose could diffuse through the cells by micrometric distances to be released in the apical region of β2 tanycytes, towards the CSF. Through this mechanism, levels of glucose would increase inside the hypothalamus, stimulating glucosensing mechanisms quickly and efficiently. KEY MESSAGES: • Glucose diffuses through the median eminence cells (β2 tanycytes), towards the hypothalamic CSF. • Glucose is transferred through a rapid "fast-track-type mechanism" via GLUT2 and GLUT6. • Through this mechanism, hypothalamic glucose levels increase, stimulating glucosensing.

Sigmoidal kinetics define porcine intestinal segregation of electrogenic monosaccharide transport systems as having multiple transporter population involvement

Kinetic characterization of electrogenic sodium-dependent transport in Ussing chambers of d-glucose and d-galactose demonstrated sigmoidal/Hill kinetics in the porcine jejunum and ileum, with the absence of transport in the distal colon. In the jejunum, a high-affinity, super-low-capacity (Ha/sLc) kinetic system accounted for glucose transport, and a low-affinity, low-capacity (La/Lc) kinetic system accounted for galactose transport. In contrast, the ileum demonstrated a high-affinity, super-high-capacity (Ha/sHc) glucose transport and a low-affinity, high-capacity (La/Hc) galactose transport systems. Jejunal glucose transport was not inhibited by dapagliflozin, but galactose transport was inhibited. Comparatively, ileal glucose and galactose transport were both sensitive to dapagliflozin. Genomic and gene expression analyses identified 10 of the 12 known SLC5A family members in the porcine jejunum, ileum, and distal colon. Dominant SGLT1 (SLC5A1) and SGLT3 (SLC5A4) expression was associated with the sigmoidal Ha/sLc glucose and La/Lc galactose transport systems in the jejunum. Comparatively, the dominant expression of SGLT1 (SLC5A1) in the ileum was only associated with Ha glucose and La galactose kinetic systems. However, the sigmoidal kinetics and overall high capacity (Hc) of transport is unlikely accounted for by SGLT1 (SLC5A1) alone. Finally, the absence of transport and lack of pharmacological inhibition in the colon was associated with the poor expression of SLC5A genes. Altogether, the results demonstrated intestinal segregation of monosaccharide transport fit different sigmoidal kinetic systems. This reveals multiple transporter populations in each system, supported by gene expression profiles and pharmacological inhibition. Overall, this work demonstrates a complexity to transporter involvement in intestinal electrogenic monosaccharide absorption systems not previously defined.

Metformin acutely lowers blood glucose levels by inhibition of intestinal glucose transport

Metformin is currently the most prescribed drug for treatment of type 2 diabetes mellitus in humans. It has been well established that long-term treatment with metformin improves glucose tolerance in mice by inhibiting hepatic gluconeogenesis. Interestingly, a single dose of orally administered metformin acutely lowers blood glucose levels, however, little is known about the mechanism involved in this effect. Glucose tolerance, as assessed by the glucose tolerance test, was improved in response to prior oral metformin administration when compared to vehicle-treated mice, irrespective of whether the animals were fed either the standard or high-fat diet. Blood glucose-lowering effects of acutely administered metformin were also observed in mice lacking functional AMP-activated protein kinase, and were independent of glucagon-like-peptide-1 or N-methyl-D-aspartate receptors signaling. [¹⁸F]-FDG/PET revealed a slower intestinal transit of labeled glucose after metformin as compared to vehicle administration. Finally, metformin in a dose-dependent but indirect manner decreased glucose transport from the intestinal lumen into the blood, which was observed ex vivo as well as in vivo. Our results support the view that the inhibition of transepithelial glucose transport in the intestine is responsible for lowering blood glucose levels during an early response to oral administration of metformin.

Inhibitory effect of luminal saccharides on glucose absorption from an adjacent jejunal site in rats: a newly described intestinal neural reflex

Nutrients in the lumen of the small intestine are sensed by special cells in the epithelial lining. The ensuing neurohumoral reflexes affect gastrointestinal absorption/secretion, motility, and vascular perfusion. To study in vivo the effect of a monosaccharide (glucose) or polysaccharide (starch) present in the jejunum on glucose absorption from an adjacent part of the intestine and investigate the possible underlying mechanisms. Using the single pass intraluminal perfusion technique, a segment of jejunum (perfusion segment) was continuously perfused with 20 mM glucose to determine glucose absorption. One hour later, a bolus of a saccharide was instilled in an isolated adjacent jejunal segment and the change in glucose absorption was monitored for a further 2 h. The contribution of neural mechanisms in this process was investigated. Instillation of glucose (20 mM or 40 mM) in either distal or proximal jejunal pouch elicited immediate and sustained inhibition of glucose absorption (a decrease by 25%; P < 0.01) from the perfused jejunal segment. Comparable inhibition was obtained with instillation of other monosaccharides or starch in the jejunal pouch. This inhibition was abolished by adding tetrodotoxin to the pouch or to the perfused jejunal segment and also by pretreatment with sympathetic blockers (guanethidine or hexamethonium) and by chemical ablation of capsaicin-sensitive primary afferent fibers. Glucose absorption within the jejunum is auto-regulated through backward and forward mechanisms. This regulation is mediated by neural reflexes involving capsaicin-sensitive afferent and sympathetic efferent fibers. These reflexes might serve to protect against hyperglycemia.

T1R2 receptor-mediated glucose sensing in the upper intestine potentiates glucose absorption through activation of local regulatory pathways

Objective

Beyond the taste buds, sweet taste receptors (STRs; T1R2/T1R3) are also expressed on enteroendocrine cells, where they regulate gut peptide secretion but their regulatory function within the intestine is largely unknown.

Methods

Using T1R2-knock out (KO) mice we evaluated the role of STRs in the regulation of glucose absorption in vivo and in intact intestinal preparations ex vivo.

Results

STR signaling enhances the rate of intestinal glucose absorption specifically in response to the ingestion of a glucose-rich meal. These effects were mediated specifically by the regulation of GLUT2 transporter trafficking to the apical membrane of enterocytes. GLUT2 translocation and glucose transport was dependent and specific to glucagon-like peptide 2 (GLP-2) secretion and subsequent intestinal neuronal activation. Finally, high-sucrose feeding in wild-type mice induced rapid downregulation of STRs in the gut, leading to reduced glucose absorption.

Conclusions

Our studies demonstrate that STRs have evolved to modulate glucose absorption via the regulation of its transport and to prevent the development of exacerbated hyperglycemia due to the ingestion of high levels of sugars.

•

The intestinal T1R2 receptor enhances glucose absorption in vivo and ex vivo.

•

Pharmacological inhibition of STRs reduces glucose flux in human intestinal preparations.

•

T1R2 regulates glucose absorption dependent on GLUT2 activity in enterocytes.

•

GLP-2 mediates the effects of T1R2 signaling through activation of enteric neurons.

•

High sucrose diet rapidly downregulates STRs leading to reduced glucose absorption.

Intestinal absorption of glucose in mice as determined by positron emission tomography

KEY POINTS

The goal was to determine the importance of the sodium-glucose cotransporter SGLT1 and the glucose uniporter GLUT2 in intestinal glucose absorption during oral glucose tolerance tests (OGTTs) in mice. Glucose absorption was determined in mice using positron emission tomography and three non-metabolizable glucose probes: one specific for SGLTs, one specific for GLUTs, and one a substrate for both SGLTs and GLUTs. Absorption was determined in wild-type, Sglt1-/- and Glut2-/- mice. Gastric emptying was a rate-limiting step in absorption. SGLT1, but not GLUT2, was important in fast glucose absorption. In the absence of SGLT1 or GLUT2, the oral glucose load delivered to the small intestine was slowly absorbed. Oral phlorizin only inhibited the fast component of glucose absorption, but it contributed to decreasing blood glucose levels by inhibiting renal reabsorption.

ABSTRACT

The current model of intestinal absorption is that SGLT1 is responsible for transport of glucose from the lumen into enterocytes across the brush border membrane, and GLUT2 for the downhill transport from the epithelium into blood across the basolateral membrane. Nevertheless, questions remain about the importance of these transporters in vivo. To address these questions, we have developed a non-invasive imaging method, positron emission tomography (PET), to monitor intestinal absorption of three non-metabolized glucose tracers during standard oral glucose tolerance tests (OGTTs) in mice. One tracer is specific for SGLTs (α-methyl-4-[18 F]fluoro-4-deoxy-d-glucopyranoside; Me-4FDG), one is specific for GLUTs (2-deoxy-2-[18 F]fluoro-d-glucose; 2-FDG), and one is a substrate for both SGLTs and GLUTs (4-deoxy-4-[18 F]fluoro-d-glucose; 4-FDG). OGTTs were conducted on adult wild-type, Sglt1-/- and Glut2-/- mice. In conscious mice, OGTTs resulted in the predictable increase in blood glucose that was blocked by phlorizin in both wild-type and Glut2-/- animals. The blood activity of both Me-4FDG and 4-FDG, but not 2-FDG, accompanied the changes in glucose concentration. PET imaging during OGTTs further shows that: (i) intestinal absorption of the glucose load depends on gastric emptying; (ii) SGLT1 is important for the fast absorption; (iii) GLUT2 is not important in absorption; and (iv) oral phlorizin reduces absorption by SGLT1, but is absorbed and blocks glucose reabsorption in the kidney. We conclude that in standard OGTTs in mice, SGLT1 is essential in fast absorption, GLUT2 does not play a significant role, and in the absence of SGLT1 the total load of glucose is slowly absorbed.

Intestinal Absorption of Fructose

Increased understanding of fructose metabolism, which begins with uptake via the intestine, is important because fructose now constitutes a physiologically significant portion of human diets and is associated with increased incidence of certain cancers and metabolic diseases. New insights in our knowledge of intestinal fructose absorption mediated by the facilitative glucose transporter GLUT5 in the apical membrane and by GLUT2 in the basolateral membrane are reviewed. We begin with studies related to structure as well as ligand binding, then revisit the controversial proposition that apical GLUT2 is the main mediator of intestinal fructose absorption. The review then describes how dietary fructose may be sensed by intestinal cells to affect the expression and activity of transporters and fructolytic enzymes, to interact with the transport of certain minerals and electrolytes, and to regulate portal and peripheral fructosemia and glycemia. Finally, it discusses the potential contributions of dietary fructose to gastrointestinal diseases and to the gut microbiome.

Chemoreceptors in the Gut

The gastrointestinal tract represents the largest interface between the human body and the external environment. It must continuously monitor and discriminate between nutrients that need to be assimilated and harmful substances that need to be expelled. The different cells of the gut epithelium are therefore equipped with a subtle chemosensory system that communicates the sensory information to several effector systems involved in the regulation of appetite, immune responses, and gastrointestinal motility. Disturbances or adaptations in the communication of this sensory information may contribute to the development or maintenance of disease. This is a new emerging research field in which perception of taste can be considered as a novel key player participating in the regulation of gut function. Specific diets or agonists that target these chemosensory signaling pathways may be considered as new therapeutic targets to tune adequate physiological processes in the gut in health and disease.

Understanding intestinal glucose transporter expression in obese compared to non-obese subjects

INTRODUCTION

The impact of Roux-en-Y gastric bypass (RYGB) on weight loss and co-morbid disease resolution is well established. However, the mechanisms underlying the procedure remain incompletely understood. Intestinal remodeling involving glucose transporters (GLUTs) may play a crucial role. Rat studies have demonstrated morphological adaptation of GLUTs within adipose and intestinal cells in association with the reprogramming of glucose metabolism. There is a limited understanding of the variations in expression amongst GLUT family receptors in the human intestine. The aim of this study was to evaluate and describe jejunal GLUT expression patterns in the obese versus non-obese.

METHODS

Tissue samples were collected from 19 adults (age ≥18) patients with morbid obesity undergoing elective RYGB. Specimens were obtained from excess jejunum removed during the stapled jejuno-jejunal anastomosis. All subjects met National Institutes of Health criteria for bariatric surgery (body mass index or BMI ≥40 or ≥35 with obesity-related comorbidities). Exclusion criteria included age less than 18, age greater than 65, patients undergoing a revision procedure, and the presence of a seizure disorder (possible association with GLUT-1 deficiency syndrome). Five samples were obtained from non-obese subjects (average BMI 26.7) without diabetes who were consenting organ donors after brain death. Samples of jejunum from non-obese individuals were obtained at the time of organ procurement. Institutional Review Board and Gift of Hope approval was obtained. Specimens underwent quantitative real-time PCR and Western blotting. Western blot densitometry was performed using Image J software. Student T test was performed using SPSS statistics software.

RESULTS

GLUT-1 and GLUT-7 expression were not detected in the jejunum of either group. No difference in expression pattern was observed for GLUT-2, GLUT-4, and GLUT-9 between the groups. Western blot band density of GLUT-5 to loading control (GADPH) mean ratio was 0.21 (SD = 0.20) in obese specimens compared to 0.56 (SD = 0.17) in non-obese. Densitometry revealed GLUT-5 levels in the jejunum of the obese were significantly lower than non-obese specimens (P < 0.05).

CONCLUSION

The absence of GLUT-1 expression in both the obese and non-obese groups is consistent with the established view of GLUT-1 being abundantly present in fetal intestine but diminished to negligible levels by adulthood. Decreased GLUT-5 expression in samples from subjects with obesity compared to non-obese samples may represent a down-regulation of gene expression amongst the obese. The differential expression of GLUT-5 suggests a possible role in obesity. Studies of GLUT family expression will aid in understanding the impact of intestinal remodeling on obesity.

Acute Effects of Sugars and Artificial Sweeteners on Small Intestinal Sugar Transport: A Study Using CaCo-2 Cells As an In Vitro Model of the Human Enterocyte

Background

The gastrointestinal tract is responsible for the assimilation of nutrients and plays a key role in the regulation of nutrient metabolism and energy balance. The molecular mechanisms by which intestinal sugar transport are regulated are controversial. Based on rodent studies, two models currently exist that involve activation of the sweet-taste receptor, T1R2/3: an indirect model, whereby luminal carbohydrates activate T1R2/3 expressed on enteroendocrine cells, resulting in the release of gut peptides which in turn regulate enterocyte sugar transport capacity; and a direct model, whereby T1R2/3 expressed on the enterocyte regulates enterocyte function.

Aims

To study the direct model of intestinal sugar transport using CaCo-2 cells, a well-established in vitro model of the human enterocyte.

Methods

Uptake of 10mM ¹⁴C D-Glucose and D-Fructose into confluent CaCo-2/TC7 cells was assessed following 3hr preincubation with sugars and artificial sweeteners in the presence and absence of the sweet taste receptor inhibitor, lactisole. Expression of the intestinal sugar transporters and sweet-taste receptors were also determined by RT-PCR.

Results

In response to short term changes in extracellular glucose and glucose/fructose concentrations (2.5mM to 75mM) carrier-mediated sugar uptake mediated by SGLT1 and/or the facilitative hexose transporters (GLUT1,2,3 and 5) was increased. Lactisole and artificial sweeteners had no effect on sugar transport regulated by glucose alone; however, lactisole increased glucose transport in cells exposed to glucose/fructose. RT-PCR revealed Tas1r3 and SGLT3 gene expression in CaCo-2/TC7 cells, but not Tas1r2.

Conclusions

In the short term, enterocyte sugar transport activities respond directly to extracellular glucose levels, but not fructose or artificial sweeteners. We found no evidence of a functional heterodimeric sweet taste receptor, T1R2/3 in CaCo-2 cells. However, when glucose/fructose is administered together there is an inhibitory effect on glucose transport possibly mediated by T1R3.

Intestinal invalidation of the glucose transporter GLUT2 delays tissue distribution of glucose and reveals an unexpected role in gut homeostasis

OBJECTIVE

Intestinal glucose absorption is orchestrated by specialized glucose transporters such as SGLT1 and GLUT2. However, the role of GLUT2 in the regulation of glucose absorption remains to be fully elucidated.

METHODS

We wanted to evaluate the role of GLUT2 on glucose absorption and glucose homeostasis after intestinal-specific deletion of GLUT2 in mice (GLUT2^ΔIEC mice).

RESULTS

As anticipated, intestinal GLUT2 deletion provoked glucose malabsorption as visualized by the delay in the distribution of oral sugar in tissues. Consequences of intestinal GLUT2 deletion in GLUT2^ΔIEC mice were limiting body weight gain despite normal food intake, improving glucose tolerance, and increasing ketone body production. These features were reminiscent of calorie restriction. Other adaptations to intestinal GLUT2 deletion were reduced microvillus length and altered gut microbiota composition, which was associated with improved inflammatory status. Moreover, a reduced density of glucagon-like peptide-1 (GLP-1) positive cells was compensated by increased GLP-1 content per L-cell, suggesting a preserved enteroendocrine function in GLUT2^ΔIEC mice.

CONCLUSIONS

Intestinal GLUT2 modulates glucose absorption and constitutes a control step for the distribution of dietary sugar to tissues. Consequently, metabolic and gut homeostasis are improved in the absence of functional GLUT2 in the intestine, thus mimicking calorie restriction.

Revisiting the physiological roles of SGLTs and GLUTs using positron emission tomography in mice

KEY POINTS

Glucose transporters are central players in glucose homeostasis. There are two major classes of glucose transporters in the body, the passive facilitative glucose transporters (GLUTs) and the secondary active sodium-coupled glucose transporters (SGLTs). In the present study, we report the use of a non-invasive imaging technique, positron emission tomography, in mice aiming to evaluate the role of GLUTs and SGLTs in controlling glucose distribution and utilization. We show that GLUTs are most significant for glucose uptake into the brain and liver, whereas SGLTs are important in glucose recovery in the kidney. This work provides further support for the use of SGLT imaging in the investigation of the role of SGLT transporters in human physiology and diseases such as diabetes and cancer.

ABSTRACT

The importance of sodium-coupled glucose transporters (SGLTs) and facilitative glucose transporters (GLUTs) in glucose homeostasis was studied in mice using fluorine-18 labelled glucose molecular imaging probes and non-invasive positron emission tomography (PET) imaging. The probes were: α-methyl-4-[F-18]-fluoro-4-deoxy-d-glucopyranoside (Me-4FDG), a substrate for SGLTs; 4-deoxy-4-[F-18]-fluoro-d-glucose (4-FDG), a substrate for SGLTs and GLUTs; and 2-deoxy-2-[F-18]-fluoro-d-glucose (2-FDG), a substrate for GLUTs. These radiolabelled imaging probes were injected i.v. into wild-type, Sglt1(-/-) , Sglt2(-/-) and Glut2(-/-) mice and their dynamic whole-body distribution was determined using microPET. The distribution of 2-FDG was similar to that reported earlier (i.e. it accumulated in the brain, heart, liver and kidney, and was excreted into the urinary bladder). There was little change in the distribution of 2-FDG in Glut2(-/-) mice, apart from a reduction in the rate of uptake into liver. The major differences between Me-4FDG and 2-FDG were that Me-4FDG did not enter the brain and was not excreted into the urinary bladder. There was urinary excretion of Me-4FDG in Sglt1(-/-) and Sglt2(-/-) mice. However, Me-4FDG was not reabsorbed in the kidney in Glut2(-/-) mice. There were no differences in Me-4FDG uptake into the heart of wild-type, Sglt1(-/-) and Sglt2(-/-) mice. We conclude that GLUT2 is important in glucose liver transport and reabsorption of glucose in the kidney along with SGLT2 and SGLT1. Complete reabsorption of Me-4FDG from the glomerular filtrate in wild-type mice and the absence of reabsorption in the kidney in Glut2(-/-) mice confirm the importance of GLUT2 in glucose absorption across the proximal tubule.

Allopurinol decreases serum uric acid level and intestinal glucose transporter-5 expression in rats with fructose-induced hyperuricemia

BACKGROUND

High fructose consumption is considered to be related to the increasing prevalence of hyperuricemia (HUA). Glucose transporters (GLUT) 2 and 5 are crucial for fructose absorption and transporter. Effects of anti-HUA drugs, allopurinol (API) and benzbromarone (BBR), on expressions of GLUT5 and GLUT2 are not evaluated.

METHOD

Wistar rats were given 10% fructose in drinking water for 60 days to induce HUA, and 5mg/kg API and 10mg/kg BBR were intragastricly treated for 30 days. Serum level of uric acid and xanthine oxidase (XOD) activity in liver were determined. Expressions of GLUT2 and GLUT5 in intestine were analyzed by immunohistochemistry staining assay and Western blot assay.

RESULTS

Treatment with API or BBR significantly decreased the serum level of uric acid in HUA rats induced by fructose. Meanwhile, API treatment significantly reduced the XOD activity in liver and GLUT5 expression in intestine. However, BBR treatment did not show inhibitory effects on hepatic XOD activity and intestinal GLUT5 expression. In addition, treatment with API or BBR did not show any effect on GLUT2 expression in intestine.

CONCLUSION

API decreases serum level of uric acid in fructose-induced HUA rats. The mechanisms are associated with suppressing XOD activity in liver to reduce uric acid production, and inhibiting GLUT5 expression in intestine to reduce fructose absorption.

In silico modelling of mass transfer & absorption in the human gut

An in silico model has been developed to investigate the digestion and absorption of starch and glucose in the small intestine. The main question we are aiming to address is the relative effect of gastric empting time and luminal viscosity on the rate of glucose absorption. The results indicate that all factors have a significant effect on the amount of glucose absorbed. For low luminal viscosities (e.g. lower than 0.1 Pas) the rate of absorption is controlled by the gastric emptying time. For viscosities higher than 0.1 Pas a 10 fold increase in viscosity can result in a 4 fold decrease of glucose absorbed. Our model, with the simplifications used to develop it, indicate that for high viscosity luminal phases, gastric emptying rate is not the controlling mechanism for nutrient availability. Developing a mechanistic model could help elucidate the rate limiting steps that control the digestion process.

A Comparison of the ATP Generating Pathways Used by S. Typhimurium to Fuel Replication within Human and Murine Macrophage and Epithelial Cell Lines

The metabolism of S. Typhimurium within infected host cells plays a fundamental role in virulence since it enables intracellular proliferation and dissemination and affects the innate immune response. An essential requirement for the intracellular replication of S. Typhimurium is the need to regenerate ATP. The metabolic route used to fulfil this requirement is the subject of the present study. For infection models we used human and murine epithelial and macrophage cell lines. The epithelial cell lines were mIC_c12, a transimmortalised murine colon enterocyte cell line that shows many of the characteristics of a primary epithelial cell line, and HeLa cells. The model macrophage cell lines were THP-1A human monocyte/macrophages and RAW 264.7 murine macrophages. Using a mutational approach combined with an exometabolomic analysis, we showed that neither fermentative metabolism nor anaerobic respiration play major roles in energy generation in any of the cell lines studied. Rather, we identified overflow metabolism to acetate and lactate as the foremost route by which S. Typhimurium fulfils its energy requirements.

Glucose transporters: cellular links to hyperglycemia in insulin resistance and diabetes

Abnormal expression and/or function of mammalian hexose transporters contribute to the hallmark hyperglycemia of diabetes. Due to different roles in glucose handling, various organ systems possess specific transporters that may be affected during the diabetic state. Diabetes has been associated with higher rates of intestinal glucose transport, paralleled by increased expression of both active and facilitative transporters and a shift in the location of transporters within the enterocyte, events that occur independent of intestinal hyperplasia and hyperglycemia. Peripheral tissues also exhibit deregulated glucose transport in the diabetic state, most notably defective translocation of transporters to the plasma membrane and reduced capacity to clear glucose from the bloodstream. Expression of renal active and facilitative glucose transporters increases as a result of diabetes, leading to elevated rates of glucose reabsorption. However, this may be a natural response designed to combat elevated blood glucose concentrations and not necessarily a direct effect of insulin deficiency. Functional foods and nutraceuticals, by modulation of glucose transporter activity, represent a potential dietary tool to aid in the management of hyperglycemia and diabetes.